In the field of hydraulic cylinders, a variety of systems have been proposed in which the displacement of the elements of the cylinder is electrically sensed and displayed or used to perform control functions. Conventional transducers have serious deficiencies owing in part to difficulties in mounting the units and the harsh environmental conditions to which they are exposed. Transducers used on hydraulic systems in the heavy equipment industry are particularly subject to damage from the severe work environment. Hydraulic cylinders are typically located in relatively unprotected areas and are typically subject to, for example, high g-forces, wide temperature variations, dust, water, debris, etc. which can result in both electrical and mechanical failure.
One attempt to provide a sensor which is particularly suitable for the heavy equipment industry uses radio frequency (RF) signals. One such system is disclosed in U.S. Pat. No. 4,737,705 issued Apr. 12, 1988 to Bitar, et al. Bitar transmits a ramping RF signal into a coaxial resonant cavity formed by the hydraulic cylinder. When the cylinder's resonant frequency is transmitted, the signal detected by a receiving antenna reaches a peak. The resonant frequency has a one to one relationship with the cylinder's extension. Thus, by determining the cylinder's resonant frequency, the cylinder's linear extension can be determined.
However, a hydraulic cylinder is almost never a perfect geometric shape (for example, a perfect cylinder.) This may result in portions of the linear extension of the cylinder where the position is inaccurate or totally unreadable. Such portions are called deadband.
For example, in Bitar, a pair of antennas are located at one end of the coaxial resonant cavity. There is a portion (deadband), starting from the antennas and extending out into the cylinder for which the position cannot be determined. The length of the deadband portion varies from cylinder to cylinder.
In many applications, this deadband portion is not critical either because for the given application the cylinder does not extend into the deadband portion or the accuracy is not needed. However, in some applications cylinder extension is needed for the full length of the cylinder.
The present invention is directed at overcoming one or more of the problems as set forth above.